Wow, the new pip 8.1.2 is so horribly broken that it gives us all kinds of problems in our daily work, especially in combination with devpi. No idea how that could ever get released. For everyone encountering issues with it (failing to parse requirement strings, failing to install) I recommend the following line:
$ python -m pip install pip==8.1.1 --upgrade
It will force a downgrade to 8.1.1 and you should be good to go again.